Posted: Wed Dec 12, 2012 6:21 am Post subject:
Problem with the VC-LFO1Subject description: Can't get out a good sine and a real sqr

Hi guys last week I 've build the vc-lfo1 (using tl071), the unit seems to work, but can't get out of it a real sine and a real square, in fact on the sine out i get a half sine /half tri (negative period) and on the sqr out i get a half sqr/ half tri, the output is not 10vpp but only 5vpp.

Is somebody have an experience similar to this???
I've checking all the board seems to bel all ok!
Ic's and other component seems good and weel soldered to the board!

Up to now I have found an error for R18 you put a 100K instead of an 1K which probably will lower a lot the amplitude of the square signal. Still this does not explain your main issue. I continue checking..._________________Yves

You mean it looks like a ramp with a maximum amplitude of 2.5V ?
If so it's half the amplitude it should be.
Check that you used a BC557B for Q1.
Measure the voltage on pin 8 of the 7555 (did you use a CMOS version of the 555 as required ?).
Check that pin 1 of the 7555 is at 0V.
Have you tried changing the 7555 ?_________________YvesLast edited by yusynth on Tue Jan 01, 2013 10:40 am; edited 1 time in total

Yes it'a 2,5v ramp,
Q1 is right,
the ic is a TLC555CP from texas instrument, should be the right one.
pin1 is 0v, pin8 is 14,78v (measured in DC mode).
Don't have another to try soon as i can a will find another one!
Thanks for all Ives.!

TLC555 is OK. The voltages are OK execpt for the zmplitude of the ramp that should be closer to 5V instead of 2.5V.
In the worst case (though I don't like the idea) you may change the value of R9 to 47K. This would fix the rest of the waves._________________Yves

Once again what is the signal at pin 7 of U2 (TL074). It should be there and unchanged, because R9 comes after and have no effect on the core of the oscillator.

Did you try to adjust T2a after changing R9 to 47K (the resistor between pins 6,7 and 2 of U2) ? By the way perform all the trimming first for the 1µF capacitor (C1a). Do not focus on the C2b until all is working and trimmed perfectly for C1a._________________Yves

Always 2,5v .......
I've try to move the trimmer etc..... But nothing work ...... The output of the wave with r9 at47k is realy low.......
Is it possible the tl074 is broke???
I Will try to change all the ic's and look to the scope..... But thats can takee a while need to make an order....

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